The suction actually drops slightly, but the CFM doubles.
On town houses it makes a big difference, or large areas
where 2 or 3 hoses can be used, without losing any performance.
I've found not using my Hide a Hoses can make a big difference too.
I'm toying with the idea of a 2 x 3 stage booster, to keep the high
vacuum and also double cfm's.
Having said all this i very rarely use my booster.